Influence of muscle oxygenation and nitrate-rich beetroot juice supplementation on O uptake kinetics and exercise tolerance.

We tested the hypothesis that acute supplementation with nitrate (NO)-rich beetroot juice (BR) would improve quadriceps muscle oxygenation, pulmonary oxygen uptake (V˙O) kinetics and exercise tolerance (T) in normoxia and that these improvements would be augmented in hypoxia and attenuated in hyperoxia. In a randomised, double-blind, cross-over study, ten healthy males completed two-step cycle tests to T following acute consumption of 210 mL BR (18.6 mmol NO) or NO-depleted beetroot juice placebo (PL; 0.12 mmol NO). These tests were completed in normobaric normoxia [fraction of inspired oxygen (FIO): 21%], hypoxia (FIO: 15%) and hyperoxia (FIO: 40%). Pulmonary V˙O and quadriceps tissue oxygenation index (TOI), derived from multi-channel near-infrared spectroscopy, were measured during all trials. Plasma [nitrite] was higher in all BR compared to all PL trials (P < 0.05). Quadriceps TOI was higher in normoxia compared to hypoxia (P < 0.05) and higher in hyperoxia compared to hypoxia and normoxia (P < 0.05). T was improved after BR compared to PL ingestion in the hypoxic trials (250 ± 44 vs. 231 ± 41 s; P = 0.006; d = 1.13), with the magnitude of improvement being negatively correlated with quadriceps TOI at T (r = -0.78; P < 0.05). T was not improved following BR ingestion in normoxia (BR: 364 ± 98 vs. PL: 344 ± 78 s; P = 0.087, d = 0.61) or hyperoxia (BR: 492 ± 212 vs. PL: 472 ± 196 s; P = 0.273, d = 0.37). BR ingestion increased peak V˙O in hypoxia (P < 0.05), but not normoxia or hyperoxia (P > 0.05). These findings indicate that BR supplementation is more likely to improve T and peak V˙O in situations when skeletal muscle is more hypoxic.